A swept wavelength system (SWS) is an optical test system that applies an optical test signal to a device under test (DUT) and records optical power data samples from the DUT. The wavelength of the optical test signal varies with time over a continuous range of wavelengths, thus the test signal is described as a swept wavelength test signal. The rate of change in wavelength may be irregular which makes it difficult to prepare SWS data having uniform time or wavelength spacing.
Existing swept wavelength systems (SWSs) collect data synchronous to an optical wavelength reference (etalon) provided by the SWS, thus the resulting data is sparse (one sample per etalon peak) as illustrated in FIG. 5. The wavelength reference occurs at known but irregular wavelength intervals λ1, λ2 and at unknown and potentially irregular time intervals s0, s1. Accordingly, a wavelength timing signal 510 triggers when the wavelength reference hits its known wavelengths λ1, λ2. Optical power samples 520 are acquired from the DUT at the triggers because at the time instant of a trigger the wavelength of the test signal applied to the DUT corresponds to one of the known wavelength references, thus the DUT response to that wavelength can be determined. The wavelength and time irregularities of wavelength references and wavelength timing signals are well known and occur because of the irregular rates of wavelength change in swept wavelength sources and the spectral characteristics of the particular etalon in the SWS, for example. Accordingly, the resulting data from existing SWSs is sparse (because it can only be measured in sync with the wavelength references) and it is irregular in wavelength and time interval (because of inherent irregularities) making use of data from existing SWSs challenging.
In SWSs such as those disclosed in U.S. Pat. Nos. 5,896,193; 6,061,124; 6,359,685; 6,552,782; and 6,940,588 which are incorporated herein by reference, optical power data is sampled from one or more devices under test (DUT) by detectors. The optical power data samples are acquired synchronously with the wavelength timing signal and hence are intrinsically correlated with the reference wavelength associated with that trigger. However, the wavelength reference and the wavelength timing signal are coarse (providing low wavelength resolution) and irregularly spaced over time. This makes it difficult to over-sample while maintaining the necessary wavelength correlation.
Existing SWSs sweeping the CL-band may include 35,600 timing pulses corresponding to 35,600 wavelength correlated optical power samples at roughly 3 picometers of resolution. Disadvantages of these SWSs include the inability to dynamically scale to higher wavelength resolutions and being unable to reduce noise without increasing measurement tine.
Turning now to FIGS. 1 and 2, an existing distributed SWS 8 is illustrated. SWSs associated with FIGS. 1 and 2 are described in greater detail in U.S. Pat. Nos. 6,940,588 and 7,079,253, which are incorporated herein by reference. In the distributed SWS 8, a front end 30 includes a tunable test laser 10 generating an optical test signal SL and a timing signal generator 14 generating a wavelength timing signal ST from a tap 12 from the test signal SL. These signals are combined by a wavelength division multiplexer (WDM) 16 and distributed 43 to a plurality of remote test stations 40a, 40b etc.
As illustrated in FIG. 2, the timing signal generator 14 uses a single fixed etalon 31 to generate a pulse train and a single Fiber Bragg Grating (FBG) 32 to generate a reference pulse, (e.g. 1520 nm) from signals tapped 12, 12a, 12b, 12c from the optical test signal SL. A second laser 34, generates the wavelength timing signal ST from electronics 33. The wavelength timing signal ST is multiplexed 16 with the optical test signal SL for transmission to the remote test stations 40a, 40b etc.
A remote test station 40a, 40b includes a wavelength division multiplex (WDM) filter 18 for separating the optical test signal SL and the wavelength timing signal ST. The wavelength timing signal ST is provided to correlator 20, which also receives sampled data from detectors 22 and 24. Several detectors 22 can be included for simultaneously detecting the output of a multi-output DUT such as a WDM filter. Generally, a large fraction (e.g. 90%) of the optical test signal output from the filter 18 is provided to the DUT 26; while a small portion (e.g. 10%) is provided to the detector 24. The optical power signal from the DUT 26 is directed to the detector 22. The actual intensity, or power, measured at the output of the DUT 26 is provided by the detector 22 to the correlator 20. Thus the correlator 20 can calculate the loss through the DUT 26 and can determine the corresponding wavelength for that particular loss calculation, in dependence upon the wavelength timing signal ST.
Referring now to FIG. 3, a schematic is illustrated for an optical test system 6 that is representative of an existing improvement to the SWS 8 described above. This SWS improvement is described in U.S. application Ser. No. 13/598,666, filed Aug. 30, 2012 which is incorporated herein by reference. Although U.S. application Ser. No. 13/598,666 could be implemented as a distributed SWS, the optical test system 6 in FIG. 3 is a benchtop embodiment of an SWS which does not use a second laser to generate the optical wavelength timing signal ST because the timing signal can be transmitted electrically. To change optical test system 6 into a distributed SWS, a second laser and a multiplexer may be included to permit the timing signals to be transmitted to remote stations via optical fiber.
In FIG. 3, a swept tunable laser 30 provides portions of a swept optical test signal 43 to two tracking stages 35 and 45, an optional wavelength reference cell 46 (such as a gas absorption cell), a reference power detector 44 and a device under test (DUT) 39. The swept optical test signal 43 from the swept tunable laser 30 is carried on an optical waveguide, such as an optical fiber. One or more splitters or taps are used to separate off the various signals, preferably with a majority of the test signal being transmitted to the DUT 39. The swept test signal is provided to the DUT 39 and the transmission response of the DUT 39 is measured by a detector 41. Alternatively, the reflection response from the DUT 39 is measured with an appropriately configured detector (not illustrated), or a second DUT detector is provided to simultaneously measure the transmission and reflection response from the DUT 39. The detector 41 converts light from the DUT 39 to an electrical signal (detector signal), which is provided to a digital signal processor (“DSP”) 50. The DSP 50 processes the detector signals from any of the detectors 36, 38, 41, 42, 44 and includes digital logic circuitry and software for signal processing. Detector 44 is an optional reference power detector which may be used to subtract out any variation in output power from the swept laser source 30 that would be a source of uncertainty in the measurement.
Tracking stages 35 and 45 include optical filter elements 37, 39 that generate a periodic optical signal as a function of input wavelength. Examples of these filters include multiple-beam fiber interferometers and Fabry-Perot filters. Fiber based Fabry-Perot filters are preferred in some embodiments because they are more easily temperature controlled and/or held at matched temperatures. Filter 37 has an FSR (free spectral range) small enough that it provides sufficient wavelength resolution. The range of FSR will depend on the application but will typically be in the range of 1 to 10 pm. Filter 39 has an FSR that is almost identical to filter 37, but is slightly longer, or slightly shorter, such that the periodic signals from the detectors of filters 37 and 39 will only become synchronized after a plurality of pulses, i.e. different number of pulses between markers depending on the particular FSR, and thereby generate a periodic beat frequency that is greater than the maximum expected mode hop of the swept laser system 6. This is illustrated in FIG. 4 using an example where the difference in FSR between the two filters is 0.3 pm. A difference larger than about 20% of the FSR between filters is not very useful. For many applications, it is better if the difference is even smaller, but it becomes progressively more expensive to make filters that have FSRs that are tightly controlled, so the practical lower limit might be more like 1% of the FSR (0.03 pm).
FIG. 4 illustrates an example of the infrequent synchronous beating of filters 37 and 39 where the filters 37, 39 are fiber-based etalons with a cavity length of 0.27 m and 0.24 m. Each of these etalons have an FSR of close to 3 pm, but beat together (align in phase) only every 30 pm (as shown by the arrows in FIG. 4).
Returning to FIG. 3, the outputs of detector 36 and 38 for the two filters 37 and 39 respectively are monitored electronically and the synchronization points (or beat pulses) are determined in the DSP block 50. In this embodiment, the DSP block includes an FPGA or other suitable hardware and software, which performs the necessary computation to determine the synchronization and may optionally include memory 52. The DSP also correlates the signal from the DUT detector 41 with the signals from the filter detectors 36 and 38 to provide an accurate wavelength for each measurement point from the DUT response signal, as is well known in the art of SWS.
The single etalon timing signal generator 14 illustrated in FIGS. 1 and 2 is commonly found in legacy SWSs. In the more recent SWSs demonstrated in FIGS. 3 and 4, the timing signal generator comprises the two filter stages 35, 45 and the reference cell 46. The timing signal is generated using the 2 fixed etalons, instead of one, and the Gas Cell 46 which provides the reference pulse in place of the Fiber Bragg Grating. The precise implementation of the timing signal generator is not relevant present disclosure; however, since the present disclosure focuses on the processing of the timing signals, not their generation, and both the legacy SWSs, and the more recent SWS both generate equivalent timing signals.
As illustrated in FIG. 5, existing SWSs provide sparse data synchronous with their etalon peaks because the etalon peaks of the SWS are the only points where the SWS knows the exact wavelength of the wavelength sweeping test signal applied to the DUT. The irregular period of the wavelength timing signal 510 corresponds to the irregular wavelength sweeping rate of the test signal source. Accordingly, existing SWSs acquire optical power samples 520 synchronously with the etalon peaks (and only once per etalon peak) because the wavelength of the test signal applied to the DUT is known only at those etalon peaks. To simplify data sampling, a wavelength timing signal 510 or synonymous signal is used in existing SWSs to translate the timing of etalon peak into a signal 510 that can coordinate the sampling of DUT data 520. In some existing SWSs, improvements have been made to identify mode hops (when the test signal undesirably skips some of the wavelengths that the SWS is trying to sweep) and adjust the wavelength and sampled data correlation. The present disclosure describes improvements to SWSs that are compatible with all these types of SWSs and many more.
Existing SWSs use each timing signal pulse as a trigger to take a measurement. This provides an array of measurements that coincide with trigger pulses which are all at known wavelengths. Increasing the resolution of an SWS requires one of two basic approaches. The first would be to simply generate the timing signal pulses at a finer resolution. There are several challenges with this approach. Firstly, the length of the etalon becomes several meters in length which becomes a major problem mechanically as this etalon requires temperature stabilization and coiling the fiber etalon too much introduces unwanted optical effects that can affect the wavelength periodicity of the etalon negatively. Secondly, it becomes progressively more and more difficult to align an edge of the etalon to the reference gas cell line. Thirdly, this approach is not backwards compatible with existing systems since this requires a change in the optics in the wavelength discriminator. The alternate approach is to continue to use the existing timing signal, but take intermediate samples between the timing pulses. Ideally this could be achieved by using a phase lock loop that would lock to the frequency of the timing signal, and then the frequency could be multiplied up to whatever frequency is needed to achieve the desired resolution. The challenge with this approach is that the speed of the laser is not constant over the entire wavelength range, making the timing pulses somewhat erratic. A phase lock loop cannot remain locked to this signal. An alternate solution is required. The present disclosure describes such a solution.
The above described existing swept wavelength systems (SWSs) are restricted to collecting irregularly time and wavelength spaced sparse optical power data because they collect data synchronous to an optical wavelength reference (etalon) which itself occurs at irregular intervals in wavelength and time. It would be advantageous to provide an SWS that can collect regularly spaced optical power data at a sampling rate higher than the wavelength reference. It would also be advantageous to provide an SWS that can provide data in regularly time or wavelength spaced intervals.